FEATURE: Add prompt=none functionality to SSO Provider protocol

This commit adds support for an optional `prompt` parameter in the
payload of the /session/sso_provider endpoint.  If an SSO Consumer
adds a `prompt=none` parameter to the encoded/signed `sso` payload,
then Discourse will avoid trying to login a not-logged-in user:

 * If the user is already logged in, Discourse will immediately
   redirect back to the Consumer with the user's credentials in a
   signed payload, as usual.

 * If the user is not logged in, Discourse will immediately redirect
   back to the Consumer with a signed payload bearing the parameter
   `failed=true`.

This allows the SSO Consumer to simply test whether or not a user is
logged in, without forcing the user to try to log in.  This is useful
when the SSO Consumer allows both anonymous and authenticated access.
(E.g., users that are already logged-in to Discourse can be seamlessly
logged-in to the Consumer site, and anonymous users can remain
anonymous until they explicitly ask to log in.)

This feature is similar to the `prompt=none` functionality in an
OpenID Connect Authentication Request; see
https://openid.net/specs/openid-connect-core-1_0.html#AuthRequest

app/controllers/session_controller.rb

@@ -59,9 +59,14 @@ def sso_provider(payload = nil, confirmed_2fa_during_login = false)
       end
 
       if data[:no_current_user]
-        cookies[:sso_payload] = payload || request.query_string
-        redirect_to path("/login")
-        return
+        if data[:prompt] == "none"
+          redirect_to data[:sso_redirect_url], allow_other_host: true
+          return
+        else
+          cookies[:sso_payload] = payload || request.query_string
+          redirect_to path("/login")
+          return
+        end
       end
 
       if request.xhr?
@@ -88,6 +93,8 @@ def sso_provider(payload = nil, confirmed_2fa_during_login = false)
     render plain: I18n.t("discourse_connect.login_error"), status: 422
   rescue DiscourseConnectProvider::BlankReturnUrl
     render plain: "return_sso_url is blank, it must be provided", status: 400
+  rescue DiscourseConnectProvider::InvalidParameterValueError => e
+    render plain: I18n.t("discourse_connect.invalid_parameter_value", param: e.param), status: 400
   end
 
   # For use in development mode only when login options could be limited or disabled.

config/locales/server.en.yml

@@ -2520,6 +2520,7 @@ en:
     email_error: "An account could not be registered with the email address <b>%{email}</b>. Please contact the site's administrator."
     missing_secret: "Authentication failed due to missing secret. Contact the site administrators to fix this problem."
     invite_redeem_failed: "Invite redemption failed. Please contact the site's administrator."
+    invalid_parameter_value: "Authentication failed due to invalid value for `%{param}` parameter. Contact the site administrators to fix this problem."
 
   original_poster: "Original Poster"
   most_recent_poster: "Most Recent Poster"

lib/discourse_connect_base.rb

@@ -7,22 +7,24 @@ class ParseError < RuntimeError
   ACCESSORS = %i[
     add_groups
     admin
-    moderator
     avatar_force_update
     avatar_url
     bio
     card_background_url
     confirmed_2fa
     email
     external_id
+    failed
     groups
     locale
     locale_force_update
     location
     logout
+    moderator
     name
     no_2fa_methods
     nonce
+    prompt
     profile_background_url
     remove_groups
     require_2fa
@@ -40,6 +42,7 @@ class ParseError < RuntimeError
     admin
     avatar_force_update
     confirmed_2fa
+    failed
     locale_force_update
     logout
     moderator

lib/discourse_connect_provider.rb

@@ -5,8 +5,17 @@ class BlankSecret < RuntimeError
   end
   class BlankReturnUrl < RuntimeError
   end
+  class InvalidParameterValueError < RuntimeError
+    attr_reader :param
+    def initialize(param)
+      @param = param
+      super("Invalid value for parameter `#{param}`")
+    end
+  end
 
   def self.parse(payload, sso_secret = nil, **init_kwargs)
+    # We extract the return_sso_url parameter early; we need the URL's host
+    # in order to lookup the correct SSO secret in our site settings.
     parsed_payload = Rack::Utils.parse_query(payload)
     return_sso_url = lookup_return_sso_url(parsed_payload)
 
@@ -32,7 +41,12 @@ def self.parse(payload, sso_secret = nil, **init_kwargs)
       raise BlankSecret
     end
 
-    super(payload, sso_secret, **init_kwargs)
+    sso = super(payload, sso_secret, **init_kwargs)
+
+    # Do general parameter validation now, after signature-verification has succeeded.
+    raise InvalidParameterValueError.new("prompt") if (sso.prompt != nil) && (sso.prompt != "none")
+
+    sso
   end
 
   def self.lookup_return_sso_url(parsed_payload)

lib/second_factor/actions/discourse_connect_provider.rb

@@ -10,7 +10,22 @@ def skip_second_factor_auth?(params)
     def second_factor_auth_skipped!(params)
       sso = get_sso(payload(params))
       return { logout: true, return_sso_url: sso.return_sso_url } if sso.logout
-      return { no_current_user: true } if !current_user
+      if !current_user
+        if sso.prompt == "none"
+          # 'prompt=none' was requested, so just return a failed authentication
+          # without putting up a login dialog and interrogating the user.
+          sso.failed = true
+          return(
+            {
+              no_current_user: true,
+              prompt: sso.prompt,
+              sso_redirect_url: sso.to_url(sso.return_sso_url),
+            }
+          )
+        end
+        # ...otherwise, trigger the usual redirect to login dialog.
+        return { no_current_user: true }
+      end
       populate_user_data(sso)
       sso.confirmed_2fa = true if @opts[:confirmed_2fa_during_login]
       { sso_redirect_url: sso.to_url(sso.return_sso_url) }

spec/requests/session_controller_spec.rb

@@ -1460,7 +1460,7 @@ def login_with_sso_and_invite(invite_key = invite.invite_key)
         expect(redirect_query["sig"][0]).to eq(expected_sig)
       end
 
-      it "it fails to log in if secret is wrong" do
+      it "fails to log in if secret is wrong" do
         get "/session/sso_provider",
             params: Rack::Utils.parse_query(@sso.payload("secretForRandomSite"))
         expect(response.status).to eq(422)
@@ -1513,6 +1513,47 @@ def login_with_sso_and_invite(invite_key = invite.invite_key)
         expect(sso2.no_2fa_methods).to eq(nil)
       end
 
+      it "fails with a nice error message if `prompt` parameter has an invalid value" do
+        @sso.prompt = "xyzpdq"
+
+        get "/session/sso_provider",
+            params: Rack::Utils.parse_query(@sso.payload("secretForOverRainbow"))
+
+        expect(response.status).to eq(400)
+        expect(response.body).to eq(
+          I18n.t("discourse_connect.invalid_parameter_value", param: "prompt"),
+        )
+      end
+
+      it "redirects browser to return_sso_url with auth failure when prompt=none is requested and the user is not logged in" do
+        @sso.prompt = "none"
+
+        get "/session/sso_provider",
+            params: Rack::Utils.parse_query(@sso.payload("secretForOverRainbow"))
+
+        location = response.header["Location"]
+        expect(location).to match(%r{^http://somewhere.over.rainbow/sso})
+
+        payload = location.split("?")[1]
+        sso2 = DiscourseConnectProvider.parse(payload)
+
+        expect(sso2.failed).to eq(true)
+
+        expect(sso2.email).to eq(nil)
+        expect(sso2.name).to eq(nil)
+        expect(sso2.username).to eq(nil)
+        expect(sso2.external_id).to eq(nil)
+        expect(sso2.admin).to eq(nil)
+        expect(sso2.moderator).to eq(nil)
+        expect(sso2.groups).to eq(nil)
+
+        expect(sso2.avatar_url).to eq(nil)
+        expect(sso2.profile_background_url).to eq(nil)
+        expect(sso2.card_background_url).to eq(nil)
+        expect(sso2.confirmed_2fa).to eq(nil)
+        expect(sso2.no_2fa_methods).to eq(nil)
+      end
+
       it "handles non local content correctly" do
         SiteSetting.avatar_sizes = "100|49"
         setup_s3